1. Field of the Invention
This invention relates to synthetic stevensite and a process for producing it. More specifically, it relates to synthetic stevensite which is formed of only three metallic components, Mg, Si and Na, has high purity, and is useful as a thickener, an adsorbent, etc., and to a process for producing it.
2. Description of the Prior Art
Synthetic hectorite has been known heretofore as a water-swellable synthetic clay mineral, as described in Japanese Laid-Open Patent Application Nos. 113115/1985 and 197671/1985. Synthetic hectorite contains lithium as a monovalent metallic component and a fluorine ion as an anionic component, and because of the deleterious effect of these components on human and other creatures, is limited in use.
Stevensite is a clay mineral having a chemical composition of the following formula EQU (Mg.sub.2.88 Mn.sub.0.02 Fe.sub.0.02)Si.sub.4 O.sub.10 (OH).sub.2.(Ca,Mg).sub.0.07 ( 1)
This mineral corresponds to a magnesium phyllosilicate [Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2 ] in which part of the magnesium component is replaced by another metal and another part of it is left vacant.
Attempts have already been made to synthesize stevensite, and for example, Clays and Clay Minerals, vol. 27, No. 4, pages 253 to 260 (1979) states that hydrothermal treatment of a sepiolite-water system yields stevensite.
As shown by the above formula (1), the naturally occurring stevensite contains colored metallic components such as iron and manganese. Furthermore, the above known synthesis method cannot completely convert sepiolite into stevensite, and has the disadvantage that a considerable amount of sepiolite remains as an impurity in the resulting product.
In the hectorite, part of the magnesium component is replaced only by a lithium atom, and therefore, the arrangement of the atoms becomes regular. Hence, hectorite is susceptible to crystallization.
On the other hand, stevensite is not regular in the arrangement of atoms and is difficult to crystallize because part of the magnesium component is replaced by another atom and part of the remainder is left vacant.
To the best of the knowledge of the present inventors, synthetic stevensite which is composed only of the three metallic components, Mg, Si and Na, and has high purity has not yet been known.